Recessed fixtures are an architecturally desireable adjunct to hung ceilings. Hung ceilings are generally designed for new and old buildings when electrical conduit and heating and ventilating ducts are placed against the ceiling of a room. In the past the space above the hung ceiling has been filled with the aforementioned equipment with air spaces around the equipment. Under these conditions the greatest danger has been overlamping. Overlamping is the condition which exists when a lamp of greater wattage than the fixture is rated for is installed. With the larger wattage lamp the heat generated is not dissappated quickly enough and the fixture temperature becomes higher than intended by the fixture designer. If the larger lamp is left in the fixture the resulting fixture temperature will rise to a point which will cause charring of plastic parts and wire insulation. Eventual deterioration of the fixture components may result in a fire. Fires from overheated fixtures have caused millions of dollars of property damage.
Recently, because of increased concern with energy conservation local and federal agencies have sponsored programs in which homes and commercial buildings are being insulated as they are built. Existing homes and commercial buildings are insulated by forcing thermal insulation into the spaces to be insulated.
Building codes require that a barrier be constructed around recessed fixtures to prevent thermal insulation from coming into contact with the fixture. For new structures the foregoing requirement is costly but feasible. The installation of barriers around recessed fixtures in existing structures is prohibitively expensive and difficult to enforce.
Materials used in the construction of recessed fixtures are thermally rated for the application. Such materials are lamps, sockets, wire and insulation to name a few. The overheating problem due to overlamping and of thermal insulation installed in contact with the fixture exists with all type of fixtures but may be particularly serious with respect to recessed incandescent fixtures.
The inventors have considered the possibility of relying upon the heat generated by the lamp in its fixture and its rate of dissipation as an indicator of safe operation. Because of the variety of fixtures and lamp sizes and the costliness of adapting each for this application the inventors have chosen the solution described by the present invention.
Safe operation of a recessed fixture depends upon the fixture dissipating a predetermined quantity of heat with its surroundings. The required heat flow is obtained by the air in contact with the fixture removing the heat. The air in contact with the fixture will heat up as it absorbs heat and the heated air, being lighter, rises allowing cooler air to flow in. This continual movement of air dependably cools the fixture. If air movement is restricted by thermal insulation a heat build up will occur resulting in a dangerous situation.